Special equipment: a spice grinder or mortar and pestle
Grind the fennel seeds and red pepper flakes together in spice grinder or with mortar and pestle until coarsely ground. Transfer to a small bowl. Mix in the Italian seasoning, garlic powder and 1 tablespoon salt. Cut each tenderloin in half crosswise to make 4 equal pieces. Rub the fennel mixture all over pork and let sit at room temperature, uncovered, 1 hour, or chill up to 8 hours.
Position a rack in center of oven preheat to 400 degrees F. Heat oil in a large cast iron skillet over medium high and cook onion and peppers, tossing occasionally, until they begin to soften, 5 about minutes. Stir in vinegar and season with salt. Let sit until cool enough to handle.
Using a sharp knife, cut slices across each tenderloin, about 3/4-inch apart going as deep as possible but without cutting all the way through. Stuff about a tablespoon of onion mixture in between each slit arranging the vegetables neatly so that there is little overhanging the sides of the tenderloin. Repeat with remaining pork and vegetables. Transfer the stuffed tenderloins to a rimmed baking sheet spaced evenly apart and arrange any remaining vegetables around each tenderloin. Roast until thickest part of the tenderloin reaches 140 degrees F, about 20 minutes. Put 2 slices of cheese over each tenderloin and continue to bake until the cheese is melted, about 5 minutes. Let sit 10 minutes before serving. Sprinkle with parsley.
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